This invention relates to electric resistance heating elements, and more particularly, to thermoplastic insulated resistance heating elements containing supporting substances.
Electric resistance heating elements are available in many forms. A typical construction includes a pair of terminal pins brazed to the ends of a Ni-Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide. While such conventional heating elements have been the workhorse for the heating element industry for decades, there have been some widely-recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces of a hot water tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years. What""s more, metal tubular elements present limited design capabilities, since their shape can not be significantly altered without losing performance.
As an alternative to metal elements, polymeric heating elements have been designed, such as those disclosed in U.S. Pat. No. 5,586,214. The ""214 patent describes a process of making a polymeric heater in which an inner mold is used having a plurality of threaded grooves for receiving a resistance wire. The assembly is first wound with a wire and thereafter injection molded with an additional coating of thermoplastic material, containing a large amount of ceramic powder for improving the thermal conductivity of the coating.
It has been discovered that injection molding a layer of thermoplastic material loaded with large amounts of ceramic powder can be difficult. The viscous polymeric material often fails to fill the mold details and can leave portions of resistance wire coil exposed. Additionally, there can be insufficient wetting between the over molded thermoplastic coating and the resistance wire, with minimal thermoplastic bonding between the inner mold and the over molded thermoplastic coating. This has led to failure of such elements during thermal cycling, since entrapped air and insufficient bonding create crack initiation sites. Crack initiation sites lead to stress cracks that can lead to shorts in emersion applications. Cracks and entrapped air also limit the heating elements"" ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element.
Efforts have been made to minimize hot and cold spots and insufficient bonding between layers of plastic materials having electrical resistance heaters disposed between their layers. In U.S. Pat. No. 5,389,184, for example, a pair of thermosetting composite structures are bonded together using a heating element containing a resistance heating material embedded within two layers of thermoplastic adhesive material. The two thermosetting components are permitted to cure, and then while applying pressure to the joint, electrical energy is passed through the heating element sufficient to heat the joint to above the melting temperature of the thermoplastic adhesive material. This heat fuses the layers of the thermoplastic adhesive to join the thermosetting materials together. The heating element remains within the joint after bonding and provides a mechanism to reheat the joint and reverse the bonding process in the field.
While these procedures have met with some success, there remains a need for a less expensive, and more structurally sound, electrical resistance heating element.
The present invention provides resistance heating elements containing a supporting substrate having a first surface thereon. An electrical resistance material is sewn with a thread to the supporting substrate to form a pre-determined circuit path having a pair of terminal end portions. Finally, a fusible layer is disposed over the circuit path and a portion of the supporting surface whereby the thread assists in retaining the resistance material in the pre-determined circuit path at least during the encapsulation by the fusible layer.
The sewing methods employed by this invention create element precursors which are more durable, easier to mold around, and flexible. The element precursors of this invention, because of the stitching used to hold the resistance heating wire onto the substrate, are much more efficient to produce and avoid the labor associated with setting pins in a fibrous material to hand wind the resistance heating wire. The efficiency of such a process cannot be understated, since up to 10-500 stitches per minute can be made to attach resistance heating material to any number of substrate types, including those containing glass fibers, thermoplastic and thermosetting materials. Because of the efficiency and accurate placement of the resistance wires sewn onto substrates with the present invention, multi-layered heating elements can be produced efficiently and accurately. Such multi-layered elements could include electrical resistance heating elements having different watt densities, thermocouples, heat distribution layers, insulation layers, metallic layers, thermistors, sensors, electronics, microchips, and fiber-optic devices without risking reliability.
In another aspect of the present invention, a method of making an insulated electrical component is provided in which an electrical resistance heating wire having a relatively thin cross-section is sewn to a supporting substrate with a thread to form a circuit path of an element precursor. The circuit path is then overmolded with a polymeric material whereby the sewn thread supports the electrical resistance wire during molding. Such molding techniques can include, for example, injection molding, blow molding, extrusion or rotational molding.